This present disclosure addresses operational safety-related issues present in conventional architectures of an ullage recirculating, Catalytic Inerting System (CIS). Ullage gas is the air and fuel vapor mixture that exists over the top of the liquid fuel in a fuel tank. In conventional CIS architectures, the ullage gas is recirculated, with the ullage gas being drawn from the fuel tank and reacted in a catalytic reactor. The catalytic process within the catalytic reactor causes oxygen and fuel vapor present in the ullage gas to chemically react and produce relatively inert chemical species, namely carbon dioxide and water vapor. Nitrogen, which typically is by far the largest component of the ullage gas, is inert and is a spectator in the fuel vapor and air reaction that occurs in the catalytic reactor. The products of the catalytic reaction and nitrogen thus are all inert and can be returned to the fuel tank to create an inert environment in the ullage space of the fuel tank. Because water is undesirable in a fuel tank, most of the water is removed from the inert gas stream by a condenser before the remaining inert gas stream is returned to the fuel tank.
Although the ullage space in the fuel tank will almost always contain fuel vapor in some concentration, the amount of fuel vapor typically is below the so-called “lower flammability limit” (LFL) for tanks containing jet fuel or diesel fuel and above the “upper flammability limit” (UFL) for fuel tanks containing gasoline. When the fuel vapor concentration in the ullage space is below the LFL, an insufficient quantity of fuel vapor exists in the ullage space to sustain a fire. When the fuel vapor concentration is above the UFL, the amount of fuel vapor present in the ullage is too great to sustain a fire. Accordingly, when the fuel vapor concentration in the ullage space is either below the LFL or above the UFL, it is almost impossible to burn the ullage gas—even a lit match in the ullage space generally will not cause the ullage gas to burn. Fuels developed for automotive and aircraft applications are typically outside the so-called flammability window defined by the LFL and the UFL at typical operating temperatures, and thus fuel tank fires in both automobiles and aircraft occur highly infrequently, even in a post-crash environment.
Although the ullage space in an aircraft fuel tank containing Jet A fuel almost always contains a fuel concentration below the LFL, there are situations in which a sufficient quantity of fuel vapor can be liberated into the ullage space to create an unsafe condition. Fuel vapor concentrations above the LFL can occur when the temperature of the liquid fuel is sufficiently high. For nominal Jet A fuel, this temperature is approximately 100° F. at sea level atmospheric pressure. The temperature threshold is reduced at higher altitudes in which the amount of air in the ullage space is reduced to a point where the fuel vapor present is great enough to be above the LFL at that pressure condition. The temperature threshold may also become an issue more frequently in vehicle types in which liquid fuel temperatures tend to be higher. The objective of the CIS architecture is to create an inert atmosphere in the ullage space irrespective of the initial conditions in the ullage space at system start up. For those situations in which a flammable condition exists in the ullage space before the CIS is started, that same flammable mixture may exist inside the internal flow passages of the CIS as well.
In conventional configurations, flame arrestors are included in the system upstream and downstream of the catalytic reactor as well as upstream and downstream of the fuel tank to keep a flame front from exiting the reactor or entering the fuel tank. Although the inclusion of flame arrestors in conventional systems should provide sufficient and redundant protection against a flame front leaving the reactor and entering the fuel tank under most conditions, a system architecture which precludes a flammable condition that may exist inside the catalytic reactor from communicating with the fuel tank ullage space would be desirable to provide enhanced safety.